Wednesday, October 05, 2016

The bits of Greenland that melted 20,000 years ago are the same bits that are melting today

My rather unsurprising heading above summarizes the findings reported in confusing detail below. So the effort below to make the findings alarming falls rather flat. It in fact draws attention to the lively possibility that present changes are as natural as the changes of 20,000 years ago.

I also append the Abstract of the underlying academic journal article, which is rather fun in its own way. I quote: "We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast"

In other words, they estimate that Greenland melting made sea levels rise by 1.5 meters over the last 20,000 years. That amounts to less than one ten thousandth of a meter per year. How worrying is that? You would need a microscope to see anything that small

The latest observations, reported in the journal Science Advances today, reveal that the entirety of Greenland is rising in response to a combination of Glacial Isostatic Adjustment (that is the rise of land due to ice mass loss over the last ~23 thousands years) and the Earth’s immediate elastic response to present-day ice-mass loss.

Mass loss of the Greenland Ice Sheet, the world’s second largest ice sheet, has increased dramatically over the last two decades, due to accelerated glacier flow and enhanced surface melting.

Scientists have historically found it hard to provide precise measurements of exactly how much the ice sheet has melted over a millennial time scale, and therefore how much it was contributing to global sea level rise over those time scales.

Earlier studies suggest that the basins of the southeast, east and northwest of the ice sheet have undergone profound change, contributing more than 77 per cent of the total ice loss to the ocean over the last century, specifically between 1900 and 1981.

Using data from the Greenland GPS Network, a team of researchers including scientists from the Technical University of Denmark (DTU) and the University of Bristol, recently found that these regions actually contributed about 40 per cent to ice mass loss over thousands of years.

Lead author Professor Shfaqat Abbas Khan from DTU, and colleagues, measured the rise of land masses that were once weighed down by ice sheets, known as Glacial Isostatic Adjustment (GIA). They found large GIA uplift rates of over 12 millimeters per year in southeast Greenland.

The results indicate that these basins alone have contributed to an ice mass loss corresponding to global sea level rise of 1.5 m.

Co-author Professor Jonathan Bamber from the University of Bristol’s School of Geographical Sciences, said: “It’s notoriously difficult to estimate the long term past contribution of Greenland to sea level rise. Our new results provide a unique insight into the millennial timescale contribution of the ice sheet. We find a remarkable similarity between how the ice sheet behaved in the past and what it is doing now, with potential implications for future mass loss trends.”

Professor Khan added: “It seems likely, therefore, that further destabilization of these ice sheet regions will continue to be the source of Greenland’s contribution to sea-level rise in the future.”

Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin-wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year.